Thermoplastics resins are typically formed by extruding the material through a through a die which shapes the material. The raw material is placed in a hopper where it is moved into and through a chamber by a screw. The mechanical action of the feeder screw heats and feeds the material through chamber which is typically in the shape of a barrel. The heated and compressed material is forced through the die at the discharge end of the extruder. The extruded part assumes the cross-sectional shape of a die.
In a single screw extruder, the screw typically has an external diameter corresponding to outer edges of the helical threads or flights. The flights may be under sized in relation to the barrel in the feed or hopper area so as to reduce feedthroat friction. Typically, a single keyway in the shaft area of the screw matches a keyway in a sleeve portion of a drive shaft so that a key positioned in the matching keyways is utilized to transmit rotational movement from the drive shaft to the screw. The key is a separate piece of rectangular metal which fits into the matching keyways. A disadvantage attendant with the use of keys is that they may be lost or misplaced. Also, worn keys can contribute to screw wobble.
As the viscosity of the thermoplastic material increases, it has been observed, that in single screws of the above described type, radial screw movement or wobble may develop. Clearance in the feed area between the outside diameter of the screw and the inside of the barrel permits movement of screw. As the screw wears, metal may be removed between the exterior of the screw and the inside of the barrel. This creates additional clearance which may cause even more wobble or radial movement.
Hence, it is desirable, to reduce radial movement without reducing clearance in the feed screw area which clearance is desirable for extruding higher viscosity materials. Reduction in radial movement beneficially reduces screw wear and contamination of the finished product with residual metal particles from the screw or barrel. Reduction in radial movement also helps maintain tighter tolerances between the screw and the barrel and can result in higher feed rates. Any reduction in radial movement can result in a more efficient powder feed and less wear on single screw extruders.